The use of the tension foil mask and a flat faceplate provides many advantages and benefits in comparison with the conventional curved or domed shadow mask. Chief among these is a greater power-handling capability which makes possible as much as a three-fold increase in brightness. The conventional curved shadow mask, which is not under tension, tends to "dome" in high-brightness picture areas where the intensity of electron bombardment is greatest. Color impurities result as the mask moves closer to the faceplate. Being under high tension, the tension foil mask does not dome or otherwise move in relation to the faceplate. Therefore, it has greater brightness potential while maintaining color purity.
The tension foil shadow mask is a part of the cathode ray tube front assembly, and is located in close adjacency to the faceplate. The front assembly comprises the faceplate with its deposits of light-emitting phosphors, a shadow mask, and support means for the mask. As used herein, the term "shadow mask" means an apertured metallic foil which may have a thickness, by way of example, of about one mil or less. The mask must be supported in high tension a predetermined distance from the inner surface of the cathode ray tube faceplate. This distance is known as the "Q-distance." The high tension may be in the range of 20 to 40 kpsi. As is well known in the art, the shadow mask acts as a color-selection electrode, or parallax barrier, which ensures that each of the three color beams lands only on its assigned phosphor deposits.
The requirements for the support means for the shadow mask are stringent. As has been noted, the shadow mask must be mounted under high tension. The mask support means must be of high strength so that the mask is held immovable. An inward movement of the mask of as little as one-tenth of a mil is significant in that guard band may be expended. Also, the shadow mask support means must be of such configuration and material composition as to be compatible with the means to which it is attached. As an example, if the support means is attached to glass such as the inner surface of the faceplate, the support means must have about the same thermal coefficient of expansion as that of the glass. The support means must provide a suitable surface for mounting the mask. Also, the support means must be of a composition such that the mask can be welded onto it by electrical resistance welding or by laser welding. The support surface preferably is of such flatness that no voids can exist between the metal of the mask and the support structure to prevent the intimate metal-to-metal contact required for proper welding.
A tension mask registration and supporting system is disclosed by Strauss in U.S. Pat. No. 4,547,696 of common ownership herewith. A frame dimensioned to enclose the screen comprises first and second space-apart surfaces. A tensed foil shadow mask has a peripheral portion bonded to a second surface of the frame. The frame is registered with the faceplate by ball-and-groove indexing means. The shadow mask is sandwiched between the frame and a stabilizing or stiffening member. When the system is assembled, the frame is located between the sealing lands of the faceplate and a funnel, with the stiffening member projecting from the frame into the funnel. While the system is feasible and provides an effective means for holding a mask under high tension and rigidly planoparallel with a flat faceplate, weight is added to the cathode ray tube, and additional process steps are required in manufacture.
There exists in the marketplace today a color tube that utilizes a tensed shadow mask. The mask is understood to be placed under high tension by purely mechanical means. Specifically, a very heavy mask support frame is compressed prior to and during affixation of the mask to it. Upon release of the frame, restorative forces in the frame cause the mask to be placed under high residual tension. During normal tube operation, electron beam bombardment causes the mask to heat up and the mask tension to be reduced. An upper limit is placed on the intensity of the electron beams that may be used to bombard the screen without causing the mask to relax completely and lose its color selection capability. The upper limit has been found to be below that required to produce color pictures of the same brightness as are produced in tubes having non-tensed shadow masks. For descriptions of examples of this type of tube, see U.S. Pat. No. 3,683,063 to Tachikawa.
Other prior art include: Lerner--U.S. Pat. No. 4,087,717; Dougherty--U.S. Pat. No. 4,045,701; Palac--U.S. Pat. No. 4,100,451; Law--U.S. Pat. No. 2,625,734; Steinberg et al--U.S. Pat. No. 3,727,087; Schwartz--U.S. Pat. No. 4,069,567; Moore--U.S. Pat. No. 3,894,321; Oess--U.S. Pat. No. 3,284,655; Hackett--U.S. Pat. No. 3,303,536; Hackett et al--U.S. Pat. No. 3,030,536; Vincent--U.S. Pat. No. 2,905,845; Fischer-Colbrie--U.S. Pat. No. 2,842,696; Law--U.S. Pat. No. 2,625,734; a journal article: "The CBS Colortron: A color picture tube of advanced design." Fyler et al. Proc. of the IRE, Jan. 1954. Dec. class R583.6; and a digest article: "A High-Brightness Shadow-Mask Color CRT for Cockpit Displays." Robinder et al. Society for Information Display, 1983.